[wpilibc] Return reference from GetInstance (#3247)

Improves consistency across all classes.

Affects Preferences, LiveWindow, and CameraServer.

Old commands Scheduler::GetInstance() was not updated as this is already
deprecated.

wpilibc/src/main/native/cpp/Preferences.cpp

@@ -17,9 +17,9 @@ using namespace frc;
 // The Preferences table name
 static wpi::StringRef kTableName{"Preferences"};
 
-Preferences* Preferences::GetInstance() {
+Preferences& Preferences::GetInstance() {
   static Preferences instance;
-  return &instance;
+  return instance;
 }
 
 std::vector<std::string> Preferences::GetKeys() {
@@ -143,3 +143,5 @@ Preferences::Preferences()
       NT_NOTIFY_NEW | NT_NOTIFY_IMMEDIATE);
   HAL_Report(HALUsageReporting::kResourceType_Preferences, 0);
 }
+
+Preferences::~Preferences() = default;